The present invention relates to fasteners in general and more in particular to a modified buttress thread form for fasteners.
Threaded fasteners are, of course, well known. They consist of a male threaded element, variously known as screws, bolts, or pins, and a mating female threaded element, variously known as nut or collar; a combination of male threaded element and a female threaded element is often referred to as a "fastener system." (In this specification, a male threaded element will usually be referred to as a screw and a female threaded element will be referred to as a nut.)
Common threaded fasteners have symmetrical threads: in a plane containing the axis of the fastener, the thread is symmetrical about a radius from the axis through the major diameter of the thread. The included angle between the thread flanks us usually 60.degree. .
Buttress threads avoid high stresses along the pressure flank of the thread in a radial direction, and have been used for that reason. A buttress thread has the pressure or load-bearing thread flank nearly perpendicular to the axis of the thread, and so the flank is not subject to high radial stresses. Typically, the pressure flank is at an angle of inclination of from about 1.degree. to about 5.degree. to avoid cutter interference during milling of the thread. Some inclination would be necessary to avoid dye interference in roll formed threads. Buttress threads often are used in breach assemblies of large guns, airplane propeller hubs, and columns for hydraulic presses.
The buttress thread forms I currently know have a relatively gentle sloping backside flank, typically at an angle of about 45.degree. to a radius from the axis of the thread. The thread forms of complementary male and female threads have the same axial lengths at corresponding radii, so the axial distances at the root of both are equal and the distances across the thread crests of both are equal. Thus thread failure in the standard buttress thread always depends on the weakest material; the collar is generally the weaker material, and in extreme cases thread failure will occur at the root of the collar thread and the pin thread will not fail at all. Another difficulty with the thread forms known by me is that there is no attempt to correlate thread height with fastener diameter so that the strength performance of a thread is proportional to fastener diameter. Instead, thread height is made a linear function of the distance between sucessive threads of a fastener, the "pitch," and pitch varies somewhat arbitrarily between different fastener diameters. (Strictly, there is only one thread in a fastener, but it is common to call the successive manifestations of the single thread seen in axial cross-section "threads," these manifestations will often be called "thread forms" in this specification.
The requirements of the threaded fastener systems are stringent in sophisticated threaded fastener systems used in aerospace. An important requirement of an aerospace fastener is a known and repeatable pre-load: the clamp-up force on the sheets held together by the fastener system. This pre-load correlates directly with the torque that sets the fastener. Nonetheless, a lot of the setting torque in a typical fastener system is not used in developing pre-load but instead is used in overcoming friction. A typical 60.degree. included angle thread experiences considerable friction between the threads of the collar and the pin because the load across the load-bearing flanks is greater than the pre-load because of the multiplying effect of the thread geometry, and accordingly driving load has to be sufficiently high to take into account the added friction in addition to the desired pre-load. The reduction of this parasitic friction has the advantages of reducing the driving load, the requirements of the setting tools, and increasing the accuracy of the pre-load. Fatigue life is critical in a fastener system for obvious reasons.
Many fastener systems have a thread lock to keep the collars from loosening on the pins. A form of thread lock uses deformed threads of the collar to increase friction between the collar and the pin threads in a localized area. Substantial hoop stress on the collar imparted through the threads of the pin can reduce or eliminate the effect of the thread lock because the hoop stress overcomes the deformation in the collar and plastically deforms the thread lock so that it looses its ability to perform its function.